EBME & Clinical Engineering Articles

Implanted devices

An implant is an artificial device made to replace and act as a missing biological structure. Outer shell of implants are often made of bioinert materials such as titanium. In some cases implants contain electronics e.g. artificial pacemaker and cochlear implants. In other cases medical implants has compound structure and acts as reinforcement e.g. dental implant or knee joint replacement implant. Some implants are active, as drug-eluting stents in the aorta and coronary arteries. (A stent is either an expandable wire mesh or hollow perforated tube that is inserted into a hollow structure of the body to keep it open.)

In Orthopedic surgery implants may refer to devices that are placed over or within bones to hold a fracture reduction and prosthesis would be the more appropriate term for devices that replace a part or whole of a defunct joint. In this context implants may be placed within the body (internal) or placed over the body (external).

An implant may also refer to a medication based on a drug suspended in a slow-release solid polypeptide carrier. These are usually administered by subcutaneous injection. Two types of commonly implanted electonic biomedical devices are cochlear implants and pacemakers.

Cochlear Implants

A cochlear implant is a surgically implanted electronic device that can help provide a sense of sound to a person who is profoundly deaf or severely hard of hearing.

The cochlear implant is often referred to as a bionic ear. Unlike other kinds of hearing aids, the cochlear implant doesn't amplify sound, but works by directly stimulating any functioning auditory nerves inside the cochlea with electrical impulses. (Named after the Latin word for snail shell, the cochlea is a coiled, tapered tube containing the auditory branch of the mammalian inner ear. Its core component is the Organ of Corti, the sensory organ of hearing).

External components of the cochlear implant include a microphone, speech processor and transmitter. The implant works by using the tonotopic organization of the basilar membrane of the inner ear. "Tonotopic organization" is the way the ear sorts out different frequencies so that our brain can process that information. In a normal ear, sound vibrations in the air lead to resonant vibrations of the basilar membrane inside the cochlea. High-frequency sounds (i.e. high pitched sounds) do not pass very far along the membrane, but low frequency sounds pass farther in. The movement of hair cells, located all along the basilar membrane, creates an electrical disturbance that can be picked up by the surrounding nerve cells. The brain is able to interpret the nerve activity to determine which area of the basilar membrane is resonating, and therefore what sound frequency is being heard.

In individuals with sensorineural hearing loss, hair cells are often fewer in number and damaged. Hair cell loss or absence may be caused by a genetic mutation or an illness such as meningitis. Hair cells may also be destroyed chemically by an ototoxic medication, or simply damaged over time by excessively loud noises. The cochlear implant by-passes the hair cells and stimulates the cochlear nerves directly using electrical impulses. This allows the brain to interpret the frequency of sound as it would if the hair cells of the basilar membrane were functioning properly.

An implant does not restore or create normal hearing. Instead, under the appropriate conditions, it can give a deaf person a useful auditory understanding of the environment and help them to understand speech when coupled with post-implantation therapy. According to researchers at the University of Michigan , approximately 100,000 people worldwide have received cochlear implants; roughly half are children and half adults. The vast majority are in developed countries due to the prohibitive cost of the device, surgery and post-implantation therapy.

Cochlear implants are controversial, and their introduction has seen the renewal of a century-old debate about models of deafness that often has the medical profession on one side and the Deaf community on the other. While cochlear implants have been welcomed by late-deafened adults, hearing parents of deaf children, audiologists, speech pathologists, and surgeons, the implantation of deaf children has been vigorously opposed by many from the signing Deaf community.

Pacemakers

A pacemaker (or "artificial pacemaker", so as not to be confused with the heart's natural pacemaker) is a medical device designed to regulate the beating of the heart.

The purpose of an artificial pacemaker is to stimulate the heart when either the heart's native pacemaker is not fast enough or if there are blocks in the heart's electrical conduction system preventing the propagation of electrical impulses from the native pacemaker to the lower chambers of the heart, known as the ventricles. [A ventricle is a heart chamber which collects blood from an atrium (another heart chamber that is smaller than ventricle) and pumps it out of the heart.]

A pacemaker is an electronic treatment for dangerously slow heartbeats. Without treatment, a slow heartbeat can lead to weakness, confusion, dizziness, fainting, shortness of breath and death.

Slow heartbeats (Brachycardia) can be the result of metabolic abnormalities or occur as a result of blocked arteries to the heart's conduction system. These conditions can often be treated and a normal heartbeat will resume. Slow heartbeats can also be a side effect of certain medications in which case discontinuation of the medicine or a reduction in dose may correct the problem.

But sometimes, the conduction system of the heart becomes irreversibly damaged for any one of a number of reasons. And some people require medications that cause slow heartbeats as a side effect in order to prevent other serious problems. Since there is no medication that one can take on chronic basis to speed up the heart rate, a pacemaker is the only solution.

Paced rhythm with single failure to capture

Fortunately, having a pacemaker implanted is only a minor surgical procedure. This is not open-heart surgery. After a pacemaker is implanted, most people resume their previous lifestyle with little or no limitations.

The procedure is performed with mild sedation and a local anaesthetic. Patients are not put to sleep. A 2-inch incision is made parallel to and just below a collarbone. Pacer wires are then inserted into a vein that lies just under the collarbone and advanced through that vein under (X ray) fluoroscopic guidance into the heart. The other end of the pacer wires are connected to a "generator" that is implanted under the skin beneath the collarbone. This generator is about half an inch deep and one and a half inches wide. The skin is then sutured closed and the patient leaves the hospital later that same day or the following day. Incisional pain is mild and transient. It is possible to feel the pacer generator under the skin and a slight deformaty of the skin can be visually noticed.

Patients may not shower for a week after the procedure to keep the incision dry and should avoid excessively exerting the arm on the side the pacer was placed for that week.

After a week, the patient may resume their prior lifestyle without limitation. Household appliances do not interfere with modern day pacemakers. However, mobile phones may. These should be kept 12 inches away from the pacemaker when on-preferably at the ear on the opposite side of the pacemaker. A mobile phone should never be left in a pocket overlying the pacemaker.

Pacemakers are programmed to suit the patients specific requirements using a pacemaker programmer. Patients with pacemakers should avoid powerful electromagnetic fields which may reprogram the pacemaker. MRI (magnetic resonance imaging) scans cannot be performed on patients with pacemakers for that reason.

Pacemaker programmer

The pacemaker generator contains a lithium battery and what is, essentially, a little computer. The generator can communicate with an external device (connected to a pacemaker programmer) which is placed on the skin overlying the pacemaker. Through this device, a physician can change the programming of the pacemaker to best suit the individual patient's needs and investigate the status of the pacemaker. Some pacemakers also report on the performance of the patient's heart.

Pacemakers can also be checked over the telephone. The patient places a device and a magnet over the pacemaker and the pacemaker transmits a signal over phone line that is analysed in the physicians office.

Pacemaker batteries give off warning signals when they are running low on power many months before they actually fail. This can be detected either by a telephone check or by a formal interrogation by the external device mentioned above. Pacemakers are generally checked at least every 3 months to allow plenty of time to change the generator when it is running low on power. Changing the generator simply means remaking the same incision, removing the old generator, and plugging the existing wires into the new generator. The patient goes home the same day. Most batteries last at least 5 years.

Pacemakers sense every heartbeat the patient has and only pace the heart when the patient's heart rate falls below a predetermined limit. Patients are usually completely unaware of when the pacer is pacing their heart. In some patients, the pacemaker only needs to fire very rarely because the slow heartbeat only occurs intermittently. In other patients, the heartbeat is always too slow and the pacemaker has to pace the heart all of the time. Such patients are said to be pacemaker dependent.

Another use of pacemakers is for a disease called hypertrophic obstructive cardiomyopathy. This is a disease where overgrown heart muscle blocks the egress of blood out of the heart. By altering the electrical activation pattern of the heart's muscle, pacemakers can help alleviate this problem.

There are two other investigational uses of pacemakers. To prevent abnormally fast heart rhythms from developing in the upper chambers of the heart (the atria), some researchers are experimenting with pacing the atria from two sites instead of just one. To improve the pumping ability of the lower chambers of the heart (the ventricles) when they have been damaged by some disease, researchers are experimenting with pacing those chambers from two different sites to improve the efficiency and co-ordination of their muscular contraction.